Hypoxia is a key regulator in liver cancer progression

We appreciate the interest by Cramer and Vaupel in our review article on understanding tumor cell heterogeneity and its implication for immunotherapy in liver cancer by single cell analysis.[1]Heinrich S. Craig A.J. Ma L. Heinrich B. Greten T.F. Wang X.W. Understanding tumour cell heterogeneity and its implication for immunotherapy in liver cancer using single-cell analysis.J Hepatol. 2021; 74: 700-715Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar Cramer and Vaupel[2]Cramer T. Vaupel P. Is tissue hypoxia the principal mechanism for immune evasion and malignant progression in Hepatocellular Carcinoma?.J Hepatol. 2021; 75: 735-736Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar questioned a statement in the article, "tumor diversity is triggered by hypoxia", which refers to our recent study on using single cell technologies to define tumor cell landscape and its biology,[3]Ma L. Hernandez M.O. Zhao Y. Mehta M. Tran B. Kelly M. et al.Tumor cell biodiversity drives microenvironmental reprogramming in liver cancer.Cancer Cell. 2019; 36 (418-430.e416)Abstract Full Text Full Text PDF Scopus (131) Google Scholar and raised concerns about determining hypoxic status in tumours with HIF target genes since HIF can be activated by hypoxia-independent manners. We agree in general that hypoxia-mediated signaling is complex, involving the activation of HIF-1 and HIF-2 and many other players, where HIF-1 is just one of the well-characterized players in response to hypoxia. We also agree that a bioinformatics-based analysis to model the status of hypoxia within a tumor lesion simply using HIF-1 target genes has a limitation as these findings may only provide a suggestive conceptual framework but not a literal causal relationship. However, we would like to draw the authors' attention to the evidence that aside from transcriptomics data, there is a wealth of experimental data available that are consistent with our statement, especially in the context of hepatocellular carcinoma (HCC).[4]Wilson G.K. Tennant D.A. McKeating J.A. Hypoxia inducible factors in liver disease and hepatocellular carcinoma: current understanding and future directions.J Hepatol. 2014; 61: 1397-1406Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar Hypoxia is a common feature in solid cancers.[5]Schito L. Semenza G.L. Hypoxia-inducible factors: master regulators of cancer progression.Trends Cancer. 2016; 2: 758-770Abstract Full Text Full Text PDF PubMed Scopus (389) Google Scholar A long-observed phenomenon in cancer is the spatial patterning of hypoxia and the distance to vasculature. This oxygen availability gradient creates a highly variable microenvironment for which tumor cells must generate adaptive responses for survival. There is a direct correlation between oxygen consumption by malignant cells and their distance to vascularization. This has been observed for decades and experimentally demonstrated by pressure of oxygen measurements among other approaches. It is conceivable that as tumors become more aggressive, more new blood vessels are observed in an attempt to correct for the low levels of oxygen. Physical evidence of this process beyond measuring downstream cellular factors within the tumor include the presence of micro-vascularization, for which HIF-1 has been identified as a key surrogate.[6]Brown J.M. Giaccia A.J. The unique physiology of solid tumors: opportunities (and problems) for cancer therapy.Canc Res. 1998; 58: 1408-1416PubMed Google Scholar HIF-1, a key sensor of cellular oxygen levels, is stabilized during hypoxia and regulates cellular responses by orchestrating a set of genes which may promote tumor progression by angiogenesis, immune evasion, metabolic reprogramming, metastasis, and induction of stemness-related features.[5]Schito L. Semenza G.L. Hypoxia-inducible factors: master regulators of cancer progression.Trends Cancer. 2016; 2: 758-770Abstract Full Text Full Text PDF PubMed Scopus (389) Google Scholar It is known that hypoxia can induce the alteration of gene expression patterns and cellular processes in both HIF-1 dependent and independent manners. Noticeably, hypoxia-related gene signatures are typically developed by comparing gene expression patterns between hypoxic and normoxic conditions using cultured mammalian cells or tissues. A recent study has nicely developed metagene signatures of hypoxia across cell types helpful in evaluating tumor hypoxia, which allows for the systematic study of its impact on the cancer genome across different cancer types.[7]Bhandari V. Hoey C. Liu L.Y. Lalonde E. Ray J. Livingstone J. et al.Molecular landmarks of tumor hypoxia across cancer types.Nat Genet. 2019; 51: 308-318Crossref PubMed Scopus (190) Google Scholar These signatures are not limited to HIF target genes. It should be noted that we have used hypoxia signatures, not just HIF target genes, to model the tumor hypoxic state in our study.[3]Ma L. Hernandez M.O. Zhao Y. Mehta M. Tran B. Kelly M. et al.Tumor cell biodiversity drives microenvironmental reprogramming in liver cancer.Cancer Cell. 2019; 36 (418-430.e416)Abstract Full Text Full Text PDF Scopus (131) Google Scholar While tumor progression depends on vascularization and angiogenesis, VEGFA, a key mediator of vascularization, is an important target gene of HIF-1. In our single-cell study of HCC and cholangiocarcinoma, we found VEGFA and other hypoxia-related genes were highly expressed in high diversity tumors. In addition, VEGFA is associated with microenvironmental reprogramming in tumors with high transcriptomic diversity.[3]Ma L. Hernandez M.O. Zhao Y. Mehta M. Tran B. Kelly M. et al.Tumor cell biodiversity drives microenvironmental reprogramming in liver cancer.Cancer Cell. 2019; 36 (418-430.e416)Abstract Full Text Full Text PDF Scopus (131) Google Scholar It is well known that HCC is highly heterogeneous, creating a major barrier to conventional cancer interventions and consequently leading to therapeutic failure. The greater the complexity and biodiversity of the tumor, the more likely it will fail to respond to any single targeted therapy. It is not surprising that bevacizumab and atezolizumab, the combination of VEGF and PD-L1 inhibitors, have shown the most promising results in HCC therapy and have already been included in common guidelines.[8]Finn R.S. Qin S. Ikeda M. Galle P.R. Ducreux M. Kim T.Y. et al.Atezolizumab plus bevacizumab in unresectable hepatocellular carcinoma.N Engl J Med. 2020; 382: 1894-1905Crossref PubMed Scopus (1082) Google Scholar Interestingly, PD-L1, associated with immune evasion, is also a HIF target gene.[5]Schito L. Semenza G.L. Hypoxia-inducible factors: master regulators of cancer progression.Trends Cancer. 2016; 2: 758-770Abstract Full Text Full Text PDF PubMed Scopus (389) Google Scholar Thus, targeting the common mechanism by combination therapy seems to be the key to future therapy for heterogenous tumors. This work was supported by grants ( ZIA-BC010313 , ZIA-BC010876 , ZIA BC 010877 , and ZIA BC 011870 ) from the Intramural Research Program of the Center for Cancer Research of the National Cancer Institute . All authors contributed to the literature research and writing of the letter. All authors have read and agreed to the published version of the letter. The authors declare no competing interests. Please refer to the accompanying ICMJE disclosure forms for further details. The following is the supplementary data to this article: Download .pdf (.24 MB) Help with pdf files Multimedia component 1 Understanding tumour cell heterogeneity and its implication for immunotherapy in liver cancer using single-cell analysisJournal of HepatologyVol. 74Issue 3PreviewOver the last decade, precision medicine and immunotherapeutic approaches have become increasingly popular in oncology. Early clinical trials reported promising results, but response rates in phase III clinical trials have been suboptimal. Knowledge gained from subsequent translational studies indicates the importance of targeting the tumour microenvironment to overcome resistance to immunotherapy. In this era of precision medicine, it is crucial to consider inter- as well as intratumoural heterogeneity. Full-Text PDF Is tissue hypoxia the principal mechanism for immune evasion and malignant progression in hepatocellular carcinoma?Journal of HepatologyVol. 75Issue 3PreviewThe advent of omics technologies and especially the possibility to analyze gene and protein expression patterns in single cells has greatly improved our understanding of intra-tumoral heterogeneity (ITH) and the dynamic nature of the tumor microenvironment (TME), both driving forces of tumor progression, therapy failure and, ultimately, patient prognosis. The outstanding review by Heinrich et al. comprehensively summarizes how single cell analyses have enabled us to better appreciate both ITH and composition of the TME of hepatocellular carcinoma (HCC). Full-Text PDF